There are many patents and literature references, too numerous to mention, which disclose techniques as well as chemicals useful to separate coal from the gangue. The most widely used of these techniques employs the sink-float principle using magnitite as the heavy medium. The literature, however, teaches literally hundreds of compounds, most halogenated hydrocarbons, as equally useful.
So far the substitution of this later genus of compounds has not had much, if any, commercial success although several pilot plants are now running using perchloroethylene. One disadvantage associated with the use of this later class of compounds is that the coal retains a considerable amount of the halogenated compound and if not removed by stringent heating its retention, on burning, causes excessive corrosion in boilers and furnaces.
Representative of recent patents disclosing equipment employed in heavy media separations are U.S. Pat. Nos. 3,348,675, 2,150,917, 2,150,899, and 2,151,578. In addition, sales and promotional literature of Otisca and McNally show licensed processes.